Pyromancer and sorcerer are the easiest starting classes, they begin with some spells which can help you in tight spots and boss fights. With sorcerer you have immediate access to some even more powerful spells from the blacksmith before New Londo Ruins. In the sorcerer's case, make sure you prioritize freeing the npc in lower undead burg to learn all the basic spells and killing the red dragon for the Drake Sword, that should get you through most of the game, until you can fight Seathe for the Moonlight Greatsword and learn advanced spells from Big Hat Logan.
My most powerful character is my initial sorcerer following a build I made up and named "Tankmage". Equipment: Giant Set +5, Bellowing Dragoncrest Ring, Ring of Steel Protection. Shields: Eagle-Crystal Ring-Black Knight. Weapon: Moonlight Greatsword +5, Sorcerer Catalyst. Stats: VIT 25-30 End: 20 Att: 23 STR: 16 INT: 75 (SL120). Spells: Crystal soul spear x2, Homing crystal soul mass x2, white dragon breath just for shits and giggles. Destroys most invaders in 3 CSS casts, can withstand 5 backstabs with no healing, also does massive melee damage, depending on invaders gear switch your shield and keep your distance, most invaders will stay in close range and try to dodge your CSS's instead of blindly charging in, just switch to Moonlight Greatsword, dual wield, then immediately back to catalyst and they will have given you some distance

Fun build, and utterly destroys any and all bosses too.
Edit: Typos